1. Field of the Invention
The present invention relates to negative-working diazotype photoreproduction and, more especially, relates to a revolutionary negative-working diazotype photoreproduction system adapted to utilize conventional diazonium salts and featuring a unique "blocked-coupler" concept.
2. Description of the Prior Art
Diazotype photoreproduction is of course a standard in the graphic arts. The touchstone of the diazography processes is the light-sensitivity of aromatic diazo salts and the fact that such salts undergo two different types of reactions: [1] replacement or decomposition, in which nitrogen is lost or evolves as nitrogen gas and some other atom or group attaches to the benzene ring in its stead; and [2] "coupling", wherein the nitrogen of the diazo function is retained and the salts react with certain couplable color-forming components, i.e., a "coupler" or "azo-coupling component", to effect formation of an azo dye species.
The photochemical sensitivity of the compounds typically employed in diazotype photographic reproduction ["photoreproduction"] materials resides in the near-ultraviolet region of the spectrum, and is centered about 400 nm [nanometers, one nm being equal to a millimicron or 10.sup.-9 meter]. Photographic diazo processes may be divided into two basic categories: positive-working processes and negative-working or reversal, processes. In the former category, the action of light causes photochemical decomposition of the diazo compound. An image is developed in the unexposed areas by the combination of the diazo compound with a "coupling" component, which is generally an aromatic amine, phenol, or aliphatic compound containing active methylene groups, to form colored oxyazo or aminoazo compounds known as azo dyes.
The positive-working material is imaged by first exposing it through a master transparency or original. The light in the exposure step must supply sufficient energy to destroy the diazo compound in the areas corresponding to the clear background of the original. High-pressure mercury vapor lamps are generally used in performing this step. That part of the diazo coating which is unprotected from the ultraviolet radiation by the image on the original becomes a colorless substance, incapable of coupling to form a dye. The unaffected diazo compound which remains in those areas where the light has not struck is able to form an azo dye by reaction with a coupling component when the medium is made alkaline. Thus, wherever there was an opaque line on the original, a dye-line appears on the copy. Positive-working, diazotype photoreproduction material is generally made alkaline, or pH adjusted, either by impregnating the material with ammonia vapors or passing it through an alkaline developing solution. See generally U.S. Pat. Nos. 1,444,469, 1,628,279, 2,217,189, 2,286,701, 2,429,249 and 2,694,009; German Patentschriften Nos. 56,606, 111,416, 487,247 and 515,205; British Pat. Nos. 234,818, 281,604 and 521,492.
In the negative-working or reversal process, a dye is formed in the exposed areas, but not in the areas protected from light. See U.S. Pat. Nos. 2,034,508 and 2,095,408; German Patentschriften Nos. 53,455 and 82,239. Thus a negative, or reversed, copy of the original transparency results. Among the processes employing diazo compounds to produce reversal copies are those employing diazosulfonates. Illustrative thereof is U.S. Pat. No. 2,854,338 to Herrick et al., wherein a neutral or acid photosensitive material is exposed to actinic light and developed in ammonia vapors. Residual diazosulfonate is removed from the unexposed areas and the background cleared by washing in water. In accordance with a more recent process, disclosed in U.S. Pat. No. 3,479,183 to Habib et al., an alkaline imaging material is exposed to actinic illuminaton to convert the diazosulfonate to an active diazonium compound which combines with a coupling component to provide a reverse dye-image. Since the generation of the imaging diazonium is thus accomplished principally by the incident radiation, powerful light sources must be used for exposing the diazotype material. After imaging, the material is acidified and the unreacted diazosulfonate is light-cleared by exposing it to overall actinic illumination, thereby forming colorless decomposition products of the diazosulfonate to produce a stable, negative dye-image against a clear background. Cf. the U.S. Pat. No. 3,713,825 to Girard; U.S. Pat. No. 1,926,322 to Van der Grinten.
Another negative-working process which utilizes diazo compounds to produce azo dye images is the diazo-oxide mode of photoreproduction.
The basic operating differences between the diazosulfonate and diazo-oxide reversal processes are as follow, reference being made to "Decomposition of o-Hydroxy-Diazonium Compounds by Light," J. DeJonge and R. Dijkstra, Recueil, 67, (1948) pp. 328-342:
[1] The irradiation product of o-hydroxy benzene diazonium compounds is a cyclopentadiene carboxylic acid while the irradiation product of benzene diazosulfonate is the structurally isomeric diazo compound itself. As a result of the light decomposition, the practical and significant difference between the negative diazosulfonate process and the diazo-oxide reversal process is the fact that a wide variety of azo dye colors may be produced from diazosulfonate coupler combinations whereas only one azo dye may be produced from each diazo-oxide. The cyclopentadiene-carboxylic acid decomposition product couples much more rapidly than phenolic, naphthol or pyrazaolone couplers [see page 335, paragraph 2 of the aforementioned article].
[2] The cyclopentadiene carboxylic acid formed as a result of decomposition under actinic light is a very active coupling component and forms red azo dyes with the diazonium compound.
[3] The fixing or prevention of image formation in the unexposed portions of diazo-oxide coating support requires that there be complete elimination of moisture or adjustment of the coated support to extremely high acid levels. This requirement need not be met with certain of the diazosulfonates.
Nonetheless, it too is well known to this art that negative-working diazography based on either diazosulfonate or diazo-oxide chemistry leaves much to be desired in terms of product imaging characteristics, namely, the diazosulfonate films require long exposure times to effect isomerization of the noncoupling trans-isomer to the reactive-cis-form needed to provide an azo dye image. Additionally, the need for impractically lengthy fixing exposures likely shall continue to militate against widespread use of this approach. Insofar as the diazo-oxide based films are concerned, same, in effect, generate coupler in situ through partial decomposition of the diazo moiety. The film is cleared by post-development exposure to destroy background diazo. Such film requires vigorous development because of the atypical course of the coupling reaction [the dye is not an arylazo structure of the usual type; rather, it is an allylazo compound akin to the aliphatic structures derived from active methylene couplers like the acetoacets]. The diazo-oxide films are also apparently quite limited in density and re-exposure schemes for enhancing image density by several successive re-exposure/cycles have been proposed. Cf. U.S. Pat. No. 4,108,664 to deBoer et al; U.S. Pat. No. 4,094,681 to Habib et al.
Yet another negative-working system features a "vesicular" film which is exposed to actinic radiation to form a latent nitrogen gas image therein from a preselected master image. The nigrogen gas evolves upon the light decomposition of the diazonium salt; upon storage, the nitrogen would slowly diffuse out of the film. However, upon heating the nitrogen expands and forms small visible bubbles or vesicles in the film. Thus, vesicular imaging is, in reality, a foaming process of sorts; same is essentially confined to single-component systems applicable to projection imaging, or microfiche/reader uses. Compare, for example, the U.S. Pat. Nos. 3,149,971, 3,355,295, 3,779,768, 3,779,774, 3,841,874 and 3,979,211.
In the U.S. Pat. No. 4,055,425 to Mustacchi, a diazotype material is described comprising a light-sensitve, water-insoluble diazoamino compound, an azo coupler, and an alkaline material, said diazamino compound, when exposed to ultraviolet light, being reactive with said azo coupler to form a visible dye product only at a pH greater than 7. Such diazotype material can be used for diazotype reproduction processes resulting in either negative or direct positive images. Such material necessitates use of rather exotic equipment and, in the negative-working mode, treatment with an external acid source is mandated, and can be corrosive to the equipment. In addition, the processing entails an extra step in the photoreproduction sequence.
"Thermal" processes are also common to the art; U.S. Pat. Nos. 3,303,028, 3,499,760, 3,695,885 and 3,899,335.
Compare also the negative-working variant disclosed in the U.S. Pat. No. 3,765,895 to Fox wherein free-radical precursors are activated by photodecomposition of a diazonium salt. This patent features the ability of diazoniums to yield a free-radical by homolytic cleavage of the C-N bond, does not entail the formation of azo dyes or ammonia development, and, indeed, such process is operable even in the absence of diazo salt.
Other variations on the theme are suggested by, e.g., U.S. Pat. Nos. 1,857,920, 2,095,408, 2,381,944, 2,416,021, 2,516,931, 2,976,145, 3,140,180, 3,307,952, 3,331,689, 3,563,744, 3,620,740, 3,769,018 and 3,793,033; British Pat. No. 975,457.
Nonetheless, a serious need continues to exist in this art for but a simple negative-working diazography process, and one which is adapted to utilize virtually any of the conventional diazo compounds typically employed to form a positive image.